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CHAPTER 24 THE ORIGIN OF SPECIES. Section A: What Is a Species?. 1.The biological species concept emphasizes reproductive isolation 2.Prezygotic and postzygotic barriers isolate the gene pools of biological species 3. The biological species concept has some major limitations

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CHAPTER 24 THE ORIGIN OF SPECIES

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CHAPTER 24 THE ORIGIN OF SPECIES

Section A: What Is a Species?

1.The biological species concept emphasizes reproductive isolation

2.Prezygotic and postzygotic barriers isolate the gene pools of biological species

3. The biological species concept has some major limitations

4. Evolutionary biologists have proposed several alternative concepts of species

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Darwin recognized that the young Galapagos Islands were a place for the genesis of new species.

The central fact that crystallized this view was the many plants and animals that existed nowhere else.

Evolutionary theory must also explain macroevolution, the origin of new taxonomic groups (new species, new genera, new families, new kingdoms)

Speciation is the keystone process in the origination of diversity of higher taxa.

Introduction

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The fossil record chronicles two patterns of speciation: anagenesis and cladogenesis.

Anagenesis is the accumulation of changes associated with the transformation of one species into another.

Fig. 24.1a

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Cladogenesis, branching evolution, is the budding of one or more new species from a parent species.

Cladogenesis promotes biological diversity by increasing the number of species.

Fig. 24.1b

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Species is a Latin word meaning “kind” or “appearance”.

Today, traditionally morphological differences have been used to distinguish species.

Today, differences in body function, biochemistry, behavior, and genetic makeup are also used to differentiate species.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


In 1942 Ernst Mayr enunciated the biological species concept to divide biological diversity.

A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species.

A biological species is the largest set of populations in which genetic exchange is possible and is genetically isolated from other populations.

1. The biological species concept emphasizes reproductive isolation

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Species are based on interfertility, not physical similarity.

For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species.

In contrast, humans haveconsiderable diversity,but we all belong to thesame species because ofour capacity to interbreed.

Fig. 24.2

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No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers.

Typically, these barriers are intrinsic to the organisms, not simple geographic separation.

Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges overlap.

Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they function before or after the formation of zygotes.

2.Prezygotic and postzygotic barriers isolate the gene pools of biological species

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Prezygotic barriers impede mating between species or hinder fertilization of ova if members of different species attempt to mate.

These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and gametic isolation.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Habitat isolation. Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating.

This is exemplified by the two species of garter snakes, in the genus Thamnophis, that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Behavioral isolation. Many species use elaborate behaviors unique to a species to attract mates.

For example, female fireflies only flash back and attract males who first signaled to them with a species-specific rhythm of light signals.

In many species,elaborate courtshipdisplays identifypotential mates ofthe correct speciesand synchronizegonadal maturation.

Fig. 24.3

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Temporal isolation. Two species that breed during different times of day, different seasons, or different years cannot mix gametes.

For example, while the geographic ranges of the western spotted skunk and the eastern spotted skunk overlap, they do not interbreed because the former mates in late summer and the latter in late winter.

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Mechanical isolation. Closely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible.

To illustrate, mechanical barriers contribute to the reproductive isolation of flowering plants that are pollinated by insects or other animals.

With many insects the male and female copulatory organs of closely related species do not fit together, preventing sperm transfer.

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Gametic isolation occurs when gametes of two species do not form a zygote because of incompatibilities preventing fusion or other mechanisms.

In species with internal fertilization, the environment of the female reproductive tract may not be conducive to the survival of sperm from other species.

For species with external fertilization, gamete recognition may rely on the presence of specific molecules on the egg’s coat, which adhere only to specific molecules on sperm cells of the same species.

A similar molecular recognition mechanism enables a flower to discriminate between pollen of the same species and pollen of a different species.

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If a sperm from one species does fertilize the ovum of another, postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult.

These barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown.

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Reduced hybrid viability. Genetic incompatibility between the two species may abort the development of the hybrid at some embryonic stage or produce frail offspring.

This is true for the occasional hybrids between frogs in the genus Rana, which do not complete development and those that do are frail.

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Reduced hybrid fertility. Even if the hybrid offspring are vigorous, the hybrids may be infertile and the hybrid cannot backbreed with either parental species.

This infertility may be due to problems in meiosis because of differences in chromosome number or structure.

For example, while a mule, the hybrid product of mating between a horse and donkey, is a robust organism, it cannot mate (except very rarely) with either horses or donkeys.

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Hybrid breakdown. In some cases, first generation hybrids are viable and fertile.

However, when they mate with either parent species or with each other, the next generation are feeble or sterile.

To illustrate this, we know that different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of hybrids die as seeds or grow into weak and defective plants.

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Reproductive barrierscan occur beforemating, betweenmating andfertilization, orafter fertilization.

Fig. 24.5

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While the biological species concept has had important impacts on evolutionary theory, it is limited when applied to species in nature.

For example, one cannot test the reproductive isolation of morphologically-similar fossils, which are separated into species based on morphology.

Even for living species, we often lack the information on interbreeding to apply the biological species concept.

In addition, many species (e.g., bacteria) reproduce entirely asexually and are assigned to species based mainly on structural and biochemical characteristics.

3.The biological species concept has some major limitations

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


Several alternative species concepts emphasize the processes that unite the members of a species.

The ecological species concept defines a species in terms of its ecological niche, the set of environmental resources that a species uses and its role in a biological community.

As an example, a species that is a parasite may be defined in part by its adaptations to a specific organism.

4.Evolutionary biologists have proposed several alternative concepts of species

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The pluralistic species concept may invoke reproductive isolation or adaptation to an ecological niche, or use both in maintaining distinctive, cohesive groups of individuals.

The biological, ecological, and pluralistic species concepts are all “explanatory” concepts - attempts to explain the very existence of a species as discrete units in the diversity of life.

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The morphological species concept, the oldest and still most practical, defines a species by a unique set of structural features.

A more recent proposal, the genealogical species concept, defines a species as a set of organisms with a unique genetic history - one tip of the branching tree of life.

The sequences of nucleic acids and proteins provide data that are used to define species by unique genetic markers.

Each species has its utility, depending on the situation and the types of questions that we are asking.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


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